In the universal mobile telecommunications system terrestrial radio access network, two potential bottlenecks in the data communication are the air interface and the transport link of the transport network. The transport link connects a radio network controller (RNC) and a Node B.
FIG. 1 schematically presents a RNC 102 connected to Node B 104 over the lub interface. The interface between the Node B 104 and a user equipment is the Uu air interface.
The transport link between the RNC 102 and Node B 104 can be a bottleneck when its capacity is smaller than the available maximal capacity over the air interface Uu. For example, a typical scenario is when the Node B 104 is connected to the RNC 102 through an E1 link having a capacity about 2 Mbps, and the available air interface Uu capacity for HSPA is significantly larger that this 2 Mbps. This means that a single radio communication device, such as user equipment, can overload the transport network (TN) at certain radio conditions.
Consequently, in the TN, packet loss may occur due to congestion or due to other reasons, i.e. data corruption.
In the Uu interface fair sharing of resources may be performed by the Uu scheduler. However, the Uu scheduler cannot handle a potential transport link bottleneck.
Hence, since the radio access network TN may be a potential bottleneck, congestion control is needed on the transport network.
There are techniques to handle TN level congestion situations, which techniques are widely deployed in running HSPA network of today.
The per-flow HSPA transport network congestion control and the active queue management (AQM) based congestion control (ABCC), are examples of congestion control techniques.
A gap in the sequence of data packets of the data packet indicates a data packet loss. This data packet loss is interpreted as congestion in the per-flow HSPA transport network congestion technique. This interpreted congestion causes a congestion action to be performed in the form of a reduction of the maximum allowed bitrate of the data packet flow. The maximum of the allowed bitrate may be reduced by as much as 50%.
In the ABCC, upon an interpreted congestion, the TCP layer is informed of TN congestion by dropping an application level IP packet.
Irrespective of the reason for the packet loss, a gap in the sequence is interpreted as congestion.